Field of the Invention
The present invention relates to an image processing apparatus for performing printing processing of image data, a method, and a storage medium storing a program.
Description of the Related Art
In an image forming apparatus, the sharpness of an output image is known to be decreased by the displacement of an ink landing position, an ink smear, an optical blur (optical dot gain), or the like. In this case, the frequency characteristic of the decrease in the sharpness of the output image is acquired, and filter processing with a reverse characteristic (sharpness recovery filter) is performed, thereby canceling the decrease in the sharpness of the output image.
The frequency characteristic of the decrease in the sharpness changes depending on the type (an inkjet printing method, an electrophotographic printing method, or the like) or model of the image forming apparatus, the type of a printing medium (paper), output conditions, the light source distribution (angle/direction) upon observation, and the like. For example, in an image forming apparatus of an inkjet printing method, an ink smear by an ink type (dye or pigment) and an optical dot gain at the time of observation change depending on a printing medium.
Japanese Patent Laid-Open No. 2003-153865 describes a technique of performing sharpness recovery processing using a sharpness recovery filter that changes depending on a printing medium, an ink type, an input device, and a portion of an object. As a technique of recovering the sharpness of an image, a PSF (Point Spread Function) is known. A blurred image can be considered as a result of convolution of a PSF in an original sharp image. For this reason, image reconstruction by deconvolution can be performed by grasping the PSF. In “Digital Image Processing Engineering” (written by Keiichi Tezuka, Tadahiro Kitahashi, and Hideo Ogawa, 1st impression of 1st edition, Nikkan Kogyo Shimbun, Jun. 15, 1985, pp. 54-57), a Wiener filter is described as an example of this technique.
An optic transfer function (OTF) that can be obtained by Fourier transformation of a PSF is information in a frequency space. This optic transfer function (OTF) can be represented by a complex number. The absolute value of the OTF, that is, the amplitude component is called an MTF (Modulation Transfer Function), and the phase component is called a PTF (Phase Transfer Function). As a method of correcting a blur of an image, a method of performing correction using the information of the optic transfer function (OTF) of an imaging optical system is known. This method is called image recovery or image reconstruction.
The sharpness degradation characteristic of an image forming apparatus may anisotropically change in accordance with the printing direction. For example, in a serial-type inkjet printing apparatus, the decrease in sharpness becomes larger in the main scanning direction than in the sub-scanning direction due to satellites, time-divisional driving, or the like. For this reason, in a method of performing isotropic sharpness recovery processing independent of the printing direction, an optimum recovery intensity cannot be set, resulting in an image degradation. For example, if sharpness recovery processing is performed at an intensity lower than the optimum intensity, the sharpness cannot sufficiently be recovered, and an image of a blurred impression is formed. If sharpness recovery processing is performed at an intensity higher than the optimum intensity, an outline portion in the image is excessively enhanced, resulting in so-called ringing.